From the first Matrix movie. Taking the red pill opens the soon-to-be Neo's eyes to the fantasy that he had been living in. Morpheus says to Neo "You have to understand that many people are not ready to be unplugged, and many of them are so inured, so hopelessly dependent on the system that they will fight to protect it."
Contrast with taking the blue pill. If Neo would have taken the blue pill, he would have stayed within the Matrix and been blind to reality.
Morpheus asks pre-Neo Keanu to take the red pill. As Neo reaches for the red pill Morpheus warns Neo "Remember, all I'm offering is the truth. Nothing more."